It is desirable that even minute leaks in liquid storage vessels be reliably detected, especially when the liquid is toxic, flammable or otherwise dangerous or highly valuable. A small change in stored liquid volume corresponds to an extremely small change in the quiescent surface level of the stored liquid. Yet a change of a few thousandths of an inch may, in a large vessel, represent a substantial leakage volume.
Prior-art liquid level detectors have attempted various methods of detecting very small changes in the height of a vertically-travelling float. Crude pivoting floats, such as used in automobile gasoline tanks, automobile carburetor float chambers, toilet tanks and the like, have heretofore been incapable of resolving the extremely small angular displacements of a pivoted float necessary for detecting a change in level of only a few thousandths of an inch reliably. The problem has primarily been the lack of a sensing and electrical transducing mechanism not only highly sensitive, but also immune to the various interfering factors, such as temperature, wave motion of the liquid surface, etc. Optical, electromagnetic and other sensing mechanisms each suffer from interfering impingement of the very medium (light, magnetism, etc.) which they are designed to sense. Furthermore, such prior-art level sensors are generally analog responsive to the particular sensed medium, ambient strength or intensity of which may vary to a greater extent than the minute variations sought to be detected.
In a pivoting float type sensor, linearity of the angular sensing transducer mechanism is important if an output is to be derived from which the actual value of level change may be calculated or displayed. Such linearity is of less importance when only a preset threshold angular displacement is to be monitored. One problem in a pivoting float is that the center of buoyancy changes position with inclination in an odd-shaped float, whereas predictability and linearity require that the radius from pivot axis to center of buoyancy remain constant at all angular displacements.
Where a shield, or stilling basin is employed to isolate a sensitive liquid level detector from an ambient surface wave motion of the liquid, it is desirable to be able to select and control the time constant, or rate of ingress and egress of the liquid between the sensitive inner portion of a stilling basin and the larger body of liquid.
Measurement errors of pivoting float type sensors also arise from motion of the vessel itself. Such a problem is evident, for example, in automobile gasoline tank gauges and in carburetor floats not mechanically compensated, during hard cornering, hill-climbing and the like. It is desirable in a sensitive leakage detector application to be able to distinguish between sensor output caused by actual change in liquid level, and that caused by change, even very minute change, in position of the vessel itself.
While a level sensor can indicate a quiescent, desired liquid level at a level somewhat above or below the middle of its measurement range, it is also desirable to be able to make a gross initial height adjustment, over a range considerably wider than the narrow sensor range, to accommodate relatively large differences in initial liquid level before the fine sensing of only thousandths of an inch change in level commences.
It is an object of the invention to provide a fluid level sensor capable of accurate, reproducible and reliable sensing of very minute liquid level changes.
Another object of the invention is to provide a fluid level sensor which senses a readily available medium, which does not rely upon intensity of the measured medium, and which is relatively impervious to variations in ambient intensity of the measured medium.
A further object of the invention is to provide a fluid level sensor relatively immune to wave motion of the liquid, and which provides for selected time constant of response to such wave motion.
Still another object of the invention is to provide a fluid level sensor having an electrical output conveniently convertible to alarm, readout or feedback-control-loop usage.
Another object of the invention is to provide a fluid level sensor having linear or predictable relationship between electrical output signal and actual fluid level in a vessel.
A still further object of the invention is to provide a fluid level sensor capable of sensing and compensating for motion of the liquid storage vessel.
Yet another object of the invention is to provide a fluid level sensor having a gross initial level setting capability considerably wider than the usual sensing range of the operating sensor.
Another object of the invention is to provide a compact, self-contained package containing sensor and related electronics, easily insertable and replaceable in existing storage tanks.